A conventional hydraulic tool, such as a wheel loader, an excavator, and a shovel, typically includes a variable displacement pump powered by an engine to push a hydraulic fluid in and out from hydraulic cylinders, so as to articulate the hydraulic tool. Such articulation of the hydraulic tool performs desired tasks, e.g., raising and/or lowering materials contained in a bucket. When the hydraulic tool is articulated a potential energy can be generated, e.g., when the materials are raised. In the conventional hydraulic tool, this potential energy is often wasted and not recovered when the hydraulic cylinders can be articulated through the potential energy, e.g., when the materials are lowered. In addition, when the hydraulic cylinders are articulated through the potential energy, the hydraulic fluid can dissipate the potential energy in form of heat and can overheat some circuit elements crossed by the hydraulic fluid, e.g., valves and/or filters.
Further, the conventional hydraulic tool may include a hydraulic circuit that requires a complex control system. This complex control system is often hydro-mechanically designed or uses linear control methods, such that the stability is essentially localized within a certain range around an operating point. To ensure the controllability of control systems with a wide range of operation, small feedback gains have to be used. Particularly, it is desired that a bandwidth of a closed loop pump control system should be sufficiently high and robust. It is very difficult to reasonably satisfy such two contradictory requirements simultaneously using a hydro-mechanical or other outer loop linear control design.
U.S. Pat. No. 8,887,499 (hereinafter the '499 patent) describes a method for overpressure control in a hydraulic system having multiple hydraulic pumps, with each hydraulic pump being connected by a respective hydraulic circuit for actuating a single respective cylinder. The method includes actuating a first variable displacement hydraulic pump which is fluidly linked by a first hydraulic circuit to a first cylinder for powering the first cylinder. According to the '499 patent, upon detecting a pressure that exceeds a predetermined threshold pressure, the flow rate of the first hydraulic pump is electronically modified to a second flow rate lower than the first flow rate. As a result, the pressure in the first hydraulic circuit is reduced to a pressure that is below the predetermined threshold pressure.